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It has issued severe rain warnings for Auckland, eastern Waikato, Coromandel Peninsula, Bay of Plenty and Gisborne for tomorrow morning, saying slips or flooding are possible.

The cyclone has already caused widespread damage in Samoa and Tonga, where it knocked buildings down and blew over trees.

"It should sideswipe the northeastern parts of Northland on Saturday morning, then get pushed off to the southeast and sideswipe Gisborne on Sunday evening. Then it will move way off to the east of us," said MetService weather ambassador Bob McDavitt.

MetService says when Wilma passes by New Zealand, it will no longer be a tropical cyclone, but will bring strong winds and heavy rain to parts of the North Island.

The heaviest rainfall is expected in Northland where 150mm to 200mm may fall within 24 hours, with rainfall rates possibly reaching 25 to 50mm per hour, which could cause surface or flash flooding, lead to slips and cause rivers and streams to rise quickly.

The rain would be accompanied by strong or gale force winds.

Northland already received 240mm of rain in 30 hours last weekend - more than double the region's average monthly rainfall of about 105mm.

Northland Regional Council civil defence manager Graeme MacDonald said he expects the storm to peak with rainfall of 30mm/hr in the early hours of Saturday.

"Once again, that will mean parts of the region could see surface flooding as rivers rise quickly."

"The rain is also expected to be accompanied by strong or gale force winds, particularly for exposed parts of the region and forecasters are also predicting heavy northeast swells and very rough seas for eastern coastal areas."

Mr Duncan said the cyclone would not affect the temperature and he has forecast between 24C and 26C for most of the upper North Island.

"The humidity may increase a little bit, but it won't get as bad as we've had recently."

Mr Duncan said the severe wind and rain would move off the country quickly with the promise of clear weather on Sunday and Monday.

"It's a quite a fast-moving weather system."

Environment Waikato emergency management officer Adam Munro says localised flooding is expected as rivers are still high after heavy rain last week.

He says the region doesn't have the cushion it had last week given it experienced a dry spell before the rain.

But Adam Munro says the good news is that expected rainfall figures aren't as high as last weekend.